This invention relates to novel optical materials, and in particular to ZnMgS-based materials and to novel photodetectors based on such materials.
There are a number of applications for UV detectors requiring a cut-off wavelength of around 300 nm or below. Examples of such applications include flame detection for fire safety, and UV astronomy. Particularly desirable would be a UV detector with a response curve that closely matched the erythemal action spectrum that describes human skin sensitivity to UV radiation.
AlGaN alloys have been proposed as the active material for solar-blind UV detectors, and a number of AlGaN Schottky barrier photodiodes have been provided that provide a direct evaluation of solar erythema or sunburning action. However, Al rich AlGaN alloys are still underdevelopment and there remain problems to be overcome. In particular the lack of a lattice-matched substrate leads to the formation of cracks, high-density dislocations and significant concentrations of shallow and deep levels, all of which limit the response time and the visible rejection power of the detectors.
Recently developed ZnSSe Schottky barrier photodiodes can be used to form visible blind UV detectors that can be grown on closely lattice-matched GaP substrates and can be provided with a cut-off wavelength tunable between about 340 nm-400 nm. There remains a need, however, for photodiodes with a still lower cut-off wavelength.
According to the present invention there is provided a UV detector comprising undoped Zn1-xMgxS as the UV responsive active material.
Preferably xxe2x89xa60.5, and more preferably still xxe2x89xa60.30.
If xxe2x89xa70.3, then the thickness of the active material must be adjusted to be kept below a critical thickness depending on the value of x. In preferred embodiments x less than 1.00, and the active material is formed as a layer of a thickness t wherein 5000 xc3x85xe2x89xa7txe2x89xa7100 xc3x85. A particularly preferred combination is wherein x=0.57 and txe2x89xa61400 xc3x85 as with these vaules the response of the active material is similar to the response of human skin to UV radiation.
One particularly preferred form of the invention is as a Schottky barrier structure, comprising in sequence: (a) a substrate layer, (b) a first electrode layer comprising n+-Zn1-yMgyS:Al, (c) an active layer of Zn1-xMgxS formed on at least a part of the upper surface of said first electrode layer, (d) a second electrode layer formed of a transparent conductive material on the upper surface of said active layer, and (e) a conducting material formed on said first electrode as an Ohmic contact (eg an Indium pellet). For example the substrate may be GaP and the second electrode gold.
According to another aspect of the present invention there is provided a method of forming a UV detector comprising the steps of:
(a) depositing a first electrode layer of n+-Zn1-yMgyS:Al on a substrate,
(b) depositing a UV responsive active layer of Zn1-xMgxS on at least a part of the upper surface of said first electrode layer,
(c) depositing a UV transparent second electrode layer on said active layer, and
(d) forming an Ohmic contact on said first electrode layer.